Default replacements, for those who might not know, are just files that switch out some bit of Maxis/EA game content for a different item made by users. Almost anything used in the game can be replaced, but some types of default replacements (skintones, eyes) are easier to make and therefore more common than others (full sets of CAS face templates.) Most default replacements can just be dropped in the Downloads folder and removed if you don't want them anymore.